The present invention is related to integrated circuit structure and processing technology and, more particularly, to antifuses in integrated circuits and their manufacture.
Antifuses are found in a growing number of integrated circuits, most of which are field programmable gate arrays (FPGAs). As the name implies, antifuses have a very high resistance (to form essentially an open circuit) in the unprogrammed ("off") state, and a very low resistance (to form essentially a closed circuit) in the programmed ("on") state. In these integrated circuits antifuses are placed at the intersections of interconnection lines which lead to different elements of the integrated circuit. By programming selected antifuses, the interconnections between the various elements of the integrated circuit are formed to define the function of the device.
In a typical antifuse structure a programming layer of antifuse material, such as amorphous silicon, silicon dioxide or silicon nitride, is sandwiched between two metal interconnection lines. Depending upon the material of each metal interconnection layer, a layer of barrier metal, such as TiW (titanium-tungsten), is placed between the programming layer and each metal interconnection layer. Barrier metal layers function to block the undesired interdiffusion of material from a programming layer, such as amorphous silicon, and material from a metal layer, such as aluminum alloy. Barrier metal layers are typically refractory metals, their intermetallics, alloys, silicides, nitrides and combinations thereof.
Various problems have been found with present day antifuses. One problem is that the programmed resistance (R.sub.ON) typically varies between 30 to 150 ohms depending upon the current used to program the antifuse. While these values are low enough for FPGA's to operate quite adequately, even lower resistance values would significantly improve performance. Thus a goal of any antifuse is to lower R.sub.ON as much as possible. Furthermore, it is highly desirable that the range of resistance values be decreased so that the performance of the FPGA be more predictable.
A second problem with present antifuses is that voltages to program the antifuses tend to vary widely. Circuit and process designs must be made to accommodate these variations, which adversely affect the cost and performance of the integrated circuit incorporating these antifuses.
The present invention solves or substantially mitigates these problems. A further advantage is that the present invention can be incorporated into existing antifuse processes without radical and expensive changes.